1. Field of the Invention
The invention relates to a process for the continuous production of SiOH-functional polysiloxanes.
2. Description of the Related Art
U.S. Pat. No. 3,668,180A describes the production of polysiloxanes by a multi-stage process using a mixture consisting of halosilanes, an organic solvent, water and alcohol.
DE102005003899A1 describes the production of polysiloxanes by reaction of halosilanes with water and alcohol in the presence of the desired polysiloxane.
U.S. Pat. No. 5,223,636 describes the production of partially alkoxylated polysiloxanes using an initial charge consisting of an alkoxysilane, to which halosilanes, water and alcohol are continuously added in such a way that the alkoxysilane-containing initial charge is always kept at boiling point.
At issue in each of these cases are complex multistage processes which comprise reacting either only halosilanes with alcohol in the presence of water and nonpolar and polar solvents, or alkoxysilanes with catalytic amounts of acid or in some cases with 0.5 to 20 wt % of chlorosilanes based on the employed amount of alkoxysilanes in the presence of water and solvent, to afford polysiloxanes, wherein after the reaction the solvent-containing product phase is removed from the aqueous phase comprising the HCl.
U.S. Pat. No. 3,489,782 teaches that it is possible to produce SiOH-functional polysiloxanes by performing a first step where chlorosilanes or mixtures of chlorosilanes and alkoxysilanes are mixed with polar solvents and a second step where this mixture and water, in an appropriate amount, is continually supplied to a reactor. The rate at which the polycondensate-containing phase is discharged from the reactor is commensurate with the rate at which reactants are supplied. By contrast, DE954198 describes a continuous loop process in which exclusively chlorosilanes or mixtures of chlorosilanes together with water and polar solvents are reacted to afford polysilicone oils, or gels no longer soluble in the aqueous phase, for example highly condensed methylpolysilicone resins, reference always being made to complete hydrolysis of the chlorosilanes.
It may generally be concluded that chlorosilane hydrolyses carried out in a batch process require not only nonpolar solvents and alcohols but also additional polar compounds which act as so-called compatibilizers between the aqueous phase and the water-insoluble solvent phase comprising the chlorosilanes. The purpose of these compatibilizers is to promote the hydrolysis/condensation reaction of the chlorosilanes dissolved in the solvent phase, with the aqueous phase.
Such compatibilizers may typically be selected, in U.S. Pat. No. 3,489,782 for example, from among carboxylic esters, for example ethyl acetate, as well as acetone for example. It will be appreciated that such esters are partially dissolved in the HCl-acidified reaction phase and thus undergo cleavage to afford corresponding amounts of alcohol.
Predominantly alkoxysilane-based continuous and/or batchwise production processes necessarily generate the corresponding alcohols in the aqueous phase.
The alcohol present in the HCl-acidified reaction phase may react with HCl to afford alkane chloride which in turn leads to high environmental pollutant levels in the wastewater resulting from the production process. These wastewater pollutant levels are measured as COD content in ppm (COD=chemical oxygen demand) and as POX content in ppm and stand for the concentration of alcohol (COD) and alkane chloride (POX) measured in the wastewater.
Wastewaters with higher concentrations of alcohol also have low flashpoints. When alcohol is already added at the start, the longer reaction time with HCl results in a markedly higher POX content.
These factors taken together result in highly complex and costly treatments of the wastewater generated by the production process.
DE 19800023 A1 describes a continuous process for producing polysiloxanes having a low SiOH content where alkoxysilanes are reacted in nonpolar solvent by addition of hydrochloric acid and optionally a small amount of chlorosilanes without polar solvent.